The present invention relates to a collimator lens for an optical fiber, which is adapted to convert divergent lights radiated from a light emitting end face of the optical fiber into a parallel luminous flux.
Utilized as light transmittive optical fiber sensor, light reflective type optical fiber sensor and light bifurcation device are a device including an optical fiber and a collimator lens attached to a light emitting end face of the optical fiber to convert divergent lights radiated from the light emitting end face of the optical fiber into a parallel luminous flux. In the device, the collimator lens is mounted on the light emitting end face of the optical fiber.
According to the collimator lens used in the conventional device, as shown in FIG. 1, a convex lens 22 formed of glass or transparent plastic material is disposed so as to position a light emitting end face of an optical fiber 21 at a focal point 23 of the convex lens 22.
Another type of the conventional device is shown in FIG. 2, wherein a rod lens 32 is disposed which has convergent distribution of refractive index, and a light emitting end face of an optical a fiber 31 is positioned at one end face of the rod lens 32. The latter type is disclosed in Japanese patent application laid open No. 59-38706. In these devices, parallel light 33 is provided at the end of the collimator lens, such as the convex lens 22 and the rod lens 32.
In the collimator for the optical fiber employing the convex lens 22 formed of transparent material such as glass and plastic material as a collimator lens shown in the firstly described conventional device with reference to FIG. 1, it is necessary to provide sufficient efficiency to convert divergent light beam radiated from the optical fiber end into parallel light beam by way of the collimator lens 22. For this purpose, the collimator lens 22 should be a convex lens subjected to highly precise machining. Accordingly, the resultant lens becomes costly. Further, it is also necessary to precisely control an angle of arrangement of the highly processed collimator lens, the position of the focal point thereof, the position of the light emitting end face of the optical fiber, and angular positional relationship therebetween. If these relative arrangements are not precisely provided, it would be impossible to provide parallel light beams.
Further, according to the second type of the conventional optical fiber collimator shown in FIG. 2, wherein employed is the rod lens 32 as a collimator lens having convergent type distribution of refractive index, the rod lens has a diameter not more than about 2 mm, otherwise the rod lens does not provide precise refractive index distribution. Therefore, the second type is not available for wide utility. .Moreover, since the rod lens having convergent refractive index distribution has small diameter, high technique is required for axial alignment between the rod lens axis and the light axis at the light emitting end face of the optical fiber. If the axes are offset from each other, it would be impossible to provide precise parallel light, moreover, the conventional rod lens cannot necessarily give enough numerical aperture (NA) to be adapted to the high NA optical fiber such as plastic optical fiber so that the conversion from the divergent light to a parallel luminous flux cannot be performed sufficiently.